Capturing carbon dioxide CO2 may be performed by a carbonation reaction in a circulating fluidized bed (CFB) using solids of metal or mineral oxides. The metal or mineral oxide acts as an absorbent of the carbon dioxide CO2, being a solid sorbent.
The reaction taking place in the CFB carbonation reactor is an exothermic reaction where the rate of reaction is dependent largely on the available surface of the solid sorbent. In addition, to satisfy reaction kinetic and equilibrium requirements of the absorption process precise control of the temperature profile is required. Therefore, reactor optimization must consider the absorption heat release, which is 178 kJ/kmol for calcium oxide CaO reacting with CO2, the resulting temperature and implications with regard to equilibrium driving force and CO2 concentration profiles.
In a CFB carbonation reactor for processing low pressure combustion flue gas, the fractions of solid materials are very low to avoid the otherwise considerable pressure drop and associated fan compression power. Reducing equipment sizes for such fluidized bed processes implies increasing fluidization velocities which may also lead to pneumatic transport operating regimes. The resulting low fractions of solid material are characterized by low overall heat transfer coefficients which ultimately depend on the fluidization gas properties. Consequently, the presently known systems for capturing carbon dioxide CO2 in CFB carbonation reactor require relatively large heat transfer surfaces which must be applied internally to remove heat from the reacting system and avoid a temperature increase of the solids sorbent to the point where the equilibrium driving force disappears and the reaction no longer occurs.
Previously known reactors remove heat from the CFB carbonation reactor according to the rate of adsorption via heat transfer area installed in the reactor. These CFB carbonation reactors include internal cooling arrangements which are placed at specified, predetermined locations. A consequence of this is that any fluctuation in process operating conditions requires adjustment in the cooling system. Such unpredictable fluctuations are disadvantageous when processes utilizing the CFB carbonation reactor waste heat are forced to absorb fluctuations due to poor CFB carbonation reactor control. Consequently, there is a demand to improve the heat transfer characteristics of the system, and to optimize the method by which heat transfer occurs to ultimately reduce heat transfer surface area and plant cost.
Moreover, a careful control of the reactor temperature is of importance for avoiding regions having low temperature and a slow reaction rate, or high temperatures and poor equilibrium driving forces. In general, poor reactor design would lead to larger reactor dimensions than otherwise required for obtaining the same carbon dioxide CO2 capture rate.
For example, considering calcium oxide CaO as sorbent, and a concentration of carbon dioxide CO2 in the carbon dioxide CO2 rich flue gas forwarded to the CFB carbonation reactor of 12% by volume, 90% of the carbon dioxide CO2 may be captured corresponding to an equilibrium carbon dioxide CO2 partial pressure at 650° C. However, if the corresponding equilibrium carbon dioxide CO2 partial pressure at 700° C. is considered, for the same flue gas, a maximum of only ˜70% capture is possible.